Data. We use the Amazon Wilds dataset, which is a multi-class sentiment classification task derived from the Amazon Reviews dataset [1]}. The input \(x\)  is the test of a review, the label \(y\)  is a corresponding star rating (from 1 to 5), and the domain \(d\)  is the identifier of the reviewer who wrote the review. The validation and test splits are comprised of both an in-distribution (ID) set and an out-of-distribution (OOD) set. The ID sets consist of reviews that are not in the training set, but are written by reviewers that are in the training set. The OOD sets consists of reviews written by a collection of reviewers that are disjoint from the training set and ID set reviewers.
